As one of performance requirements for an inkjet recording ink (inkjet ink), the formability of an image excellent in transparency, clarity and the like can be mentioned, for example. In inks for use in office-automation inkjet printers, dyes capable of forming images excellent in transparency, clarity and the like have, therefore, been used as colorants conventionally. Inkjet inks have, however, been finding utility to obtain prints to be posted outdoors, such as posters for outdoor display and prints for building materials, because of the popularization of diverse inkjet printers in recent years. Nonetheless, images recorded with dye inks, which contain dyes as colorants, involve a problem that they are prone especially to color fading or color skipping when posted. outdoors. From these circumstances, there is also an increasing demand for pigment inks, in which pigments excellent in weather (light) resistance are incorporated as colorants, in inkjet recording.
As colorants for inkjet inks, yellow, magenta, cyan and like pigments such as those to be described below are used. Among such magenta pigments and cyan pigments, medium- to high-grade pigments excellent in weather (light) resistance, such as quinacridone-based pigments and phthalocyanine-based pigments, are primarily used. Among such yellow pigments, on the other hand, C.I. Pigment Yellow 74, an insoluble azo yellow pigment, is mainly used from the aspect of production cost, production easiness, transparency and clarity, and the like. However, C.I. Pigment Yellow 74 as an azo pigment is accompanied by a problem that its weather (light) resistance is insufficient compared with the medium- and high-grade pigments used as the magenta pigments and cyan pigments.
The above-described C.I. Pigment Yellow 74 can be synthesized, for example, by a coupling reaction that a diazonium salt, which has been obtained by diazotizing an aromatic amine compound, and a coupling component are reacted. As another illustrative synthesis method of C.I. Pigment Yellow 74, an accessory-assisted coupling method is known. According to this accessory-assisted coupling method, similar to conventionally-known synthesis methods of azo pigments, a component which is similar in structure to a main raw material [an aromatic amine compound (diazonium salt) or coupling component] is added as an accessory to a solution of the diazonium salt or a grounding solution at the time of a coupling reaction, and is reacted. As a further known method, heat treatment is conducted after a coupling reaction to induce crystal growth such that pigment particles are provided with enhanced fastness. It is, however, the current situation that, whichever method is used, no C.I. Pigment Yellow 74 has been obtained yet with performance sufficient to satisfy the various performance required for a pigment for use in inkjet inks.
The present applicant has already proposed, as a technology on C.I. Pigment Yellow 74, a C.I. Pigment Yellow 74 in which the average particle size is 30 to 150 nm, the half-value width of a maximum peak or a similar peak in an X-ray diffraction spectrum is 0.3 to 0.7°, and the (average value) of the ratios of major axes to minor axes of particles is 1 to 1.5 (see Patent Document 1). This C.I. Pigment Yellow 74 is excellent in light resistance, and enables the recording of images having good transparency and clarity.